1. Field of the Invention
This invention relates to a golf ball having improved symmetry and more particularly, to a golf ball having improved aerodynamics in that it will follow the same trajectory on both impact at a pole of the ball and seam hitting and its performance does not vary with different impact points.
2. Prior Art
The flying performance of golf balls is greatly affected by the arrangement and configuration (including diameter, depth and cross-sectional shape) of dimples. Various dimple arrangements are known in the art for arranging a plurality of dimples on the ball surface in an even or dense fashion. Typical known dimple arrangements are regular polyhedral arrangements. It is also known to equally divide the hemisphere into one to seven sections, especially three to six sections from its center.
In JP-B 7875/1994 the dimple configuration is tailored such that the overall effective volume of dimples remains substantially equal between pole hitting (the spin axis is in the equator plane) and seam hitting (the spin axis is a pole-to-pole line).
Golf balls are generally molded in an axisymmetric manner by using a mold comprising a pair of mold halves, removably mating them along a parting line to define a spherical cavity therein, and introducing stock material into the cavity. The thus molded golf balls tend to have a higher degree of roundness about a pole-to-pole axis corresponding to a line connecting the apexes of the mold half cavities, but a lower degree of roundness about an axis on a plane circumscribed by a seam line corresponding to the parting plane of the mold. Because of such roundness variation, conventional golf balls exhibit different flight performance depending on the position at which the ball is hit. Such flight performance variation raises a serious problem in the game of golf wherein the Rules of Golf prescribe that "the ball shall be played as it lies, except as otherwise provided in the Rules."
More specifically, when a golf ball is hit by a club, the ball is given back spin although the number of revolutions varies with a particular type of club. The ball hitting is generally classified into pole hitting and seam hitting depending on an impact point. Reference is now made to FIG. 6(A) and 6(B) wherein a golf ball 10 has a seam line (equator line) 12 and a center 16. The pole hitting means that the ball 10 is hit at arrow 20 to give back spin about a straight line 18 connecting two diametrically opposed points 14, 14 on the seam line 12 and the center 16 as shown in FIG. 6(A). The seam hitting means that the ball 10 is hit at arrow 26 to give back spin about a straight line 24 extending perpendicular to a circular plane 22 circumscribed by the seam line 12 and passing the center 16. As previously mentioned, in the event of pole hitting shown in FIG. 6(A), the ball is susceptible to extra lift or drag since it does not define a true circle about the spin axis 18. It is thought that the surface portion which does not define a true circle may give a dimple-like effect at the land on which dimples are not provided. On the other hand, in the event of seam hitting shown in FIG. 6(B), the ball is substantially free of extra lift or drag since it is close to a true circle about the spin axis 24. In many cases, the seam line or equator line is formed as an endless (continuous) band-like land on which any dimples do not intercept. As a consequence, if the ball is simply designed such that the effect of dimples may be equal between pole hitting and seam hitting, the effect of dimples would be greater on pole hitting because of a deviation from roundness. Then on pole hitting, the golf ball receives extra lift or drag, exhibiting different flight performance than on seam hitting. This means that the flight performance varies with a particular hit position.
To produce a golf ball which is improved in symmetry in that the flight performance remains constant regardless of a particular hit position, the arrangement and configuration of dimples must be designed in consideration of the shape or roundness of the ball so as to optimize the effect of dimples. This requirement has not been fully satisfied.